A Casus Belli, sometimes abbreviated CB and meaning "Cause for War" in Latin, represents a justification for war that is recognized as legitimate by other characters. A ruler must have a valid Casus Belli in order to be able to declare war upon another ruler and can only gain from a war what the Casus Belli specifies.
The War Target is the map region which is targeted by a Casus Belli. If the Casus Belli does not target a map region then the defender's Realm Capital becomes the War Target.
The attacking side gains more War Score by occupying Holdings locates in the War Target region. After a grace period the defender will passively gain War Score by holding all Counties in the War Target region.
Each Casus Belli will cost Prestige and/or Piety. If a character cannot pay the cost it can pay with a Level of Prestige or Level of Devotion instead. Declaring war against a character's Head of Faith costs 500 Piety if the Head of Faith is Spiritual and 100 Piety if Head of Faith is Temporal.
Casus Belli costs can be removed via one of the game rules.
De Jure CBs
All Title owners have a De Jure CB towards characters who own a lower Title that belongs to the de jure territory of the higher Title. All De Jure CBs cost 100 Prestige per County in the War Target Title and have a War Contribution Share of 300 Prestige.
- If the war ends by enforcing demands the attacker will gain the War Target Title and if it's a Vassal its Liege will gain a Weak Hook on them
- If the war ends by white peace the defender keeps the War Target Title
- If the war ends by surrender the defender's keeps the War Target Title and the attacker must pay Gold
|War Target rank||Required Innovation|
|All Titles||Rightful Ownership|
The following CBs are only available to Rulers who have Clan or Tribal government or whose Faith has the Warmonger or Pursuit of Power tenets.
- If the war ends by enforcing demands the attacker will gain the War Target Title and Fame equal to the War Contribution Share
- If the war ends by white peace the defender keeps the War Target Title and the attacker loses 75 Prestige
- If the war ends by surrender the defender's keeps the War Target Title and the attacker must pay Gold and gain +25 Faction Discontent
Holy War CBs
The following CBs will cause a Holy War instead of a regular one and require the Faith to be organized and to not have the Pacifism or Dharmic Pacifism tenets and can only target rulers whose Faith is considered Hostile or Evil.
- If the war ends by enforcing demands the attacker will gain all Titles held by Rulers of different Faith and vassalize all Rulers of the same Faith in the War Target region. The attacker's faith loses the Fervor cost and the defender's faith gains the twice that amount.
- If the war ends by white peace the defender keeps all their Titles and the attacker loses 50 Piety
- If the war ends by surrender the defender's keeps all their Titles, the attacker must pay Gold and gain +25 Faction Discontent and -15 Vassal Opinion for 5 years
|War Target rank||Required Level of Devotion||Fervor cost||Piety cost (without Armed Pilgrimages tenet)||Piety cost (with Armed Pilgrimages tenet)||War Contribution Share||Once per lifetime|
|Kingdom||Paragon of Virtue||5||750||600||500|
Migrate to Pannonia
Migrate to Pannonia is a unique Casus Belli with special mechanics that can only be used once per game by characters of Táltosist faith who make the Prepare to Cross the Carpathians decision against characters in the Pannonian Basin region. If the Hungarian Migration game rule is set to Immediate the game will start with the casus belli being used by Grand Prince Álmos Ügyekfi of the Mogyër Confederation against King Boris of Bulgaria.
If the war ends in white peace the defender keeps his or her titles and if it ends in surrender the attacker's factions will also gain discontent. If the war ends by enforcing demands however it will have the following effects:
- Attacker gains all titles held by the defender in the Pannonian Basin region
- Attacker gains the Kingdom of Hungary title, which replaces Kingdom of Pannonia
- Any other kingdom title held by the attacker is destroyed
- Any held title that is not within the de jure realm of Kingdom of Hungary is given to another character
- All vassals that are not within the de jure realm of Kingdom of Hungary become independent
- All controlled counties convert to Mogyër culture and Táltosist faith
- All previously controlled counties convert to Pecheneg culture
When the Casus Belli is used it will spawn the following army for as long as the attacking character lives:
Varangian Adventure is a unique Casus Belli with special mechanics that can only be used during the Tribal Era on Counties and Duchies owned by characters from a different Culture Group and costs 1000 Prestige. To be used it requires the Varangian Adventurers innovation, which is only available to North Germanic cultures and requires the Northern Lords DLC. The Casus Belli can be used multiple times but each use will cost 1000 more Prestige than the previous one. It cannot be used by Kings or Emperors.
If the war ends in white peace the defender keeps his or her titles and if it ends in surrender it must also pay Gold and Prestige to the attacker. If the war ends by enforcing demands however it will have the following effects:
- All titles held by the attacker are destroyed or given to other characters
- All vassals are given independence
- Attacker gains the War Target title and all titles that are de jure part of it
- Attacker gains the Adventurer trait
- Attacker gains 200 Gold
- If the War Target title was a County and its de jure Duchy does not exist or is part of the defender's realm the Duchy is given to the attacker
When the Casus Belli is used for the first time it will spawn the following for the duration of the war the following army based on the number of owned Counties of the same Faith and Culture in the region of Mainland Scandinavia:
AI Rulers will only use the Casus Belli if all their held titles are located in the Mainland Scandinavia region. If the Scandinavian Adventurers game rule is not set to Apocalyptic then they also require the Ambitious trait, as well as at least two of the following traits: Adventurer, Arrogant, Brave, Greedy.